This invention relates to a method of discoloration of metallic compounds which comprises irradiating laser rays on a metallic compound selected from the group consisting of oxides, hydroxides, sulfides, carbonates, chromates and titanates of metal elements.
In general, metallic compounds, especially inorganic metal compounds such as oxides and sulfides, are so stable that even when they are allowed to stand still in air maintained at room temperature, no change in the composition is brought about by decomposition or reduction and therefore, no discoloration occurs in these metallic compounds.
Methods for discoloring specific areas of a molded article of a metallic compound have heretofore been proposed. For instance, the specification of U.S. Pat. No. 3,138,547 teaches a method comprising passing an electric current to a metallic compound, for example zinc oxide, to which semi-conductivity is given, to thereby cause discoloration in the metallic compound as a result of reduction of such metal compound.
However, this method is still defective in the following points. Namely, the objective substance is limited to a specific metallic compound which has been made semiconductive, and special manufacturing steps are necessary for forming such semiconductive metallic compounds. Further, since it is necessary to contact the metallic compound with a solid electrode substance so as to cause discoloration, the scanning rate of a recording stylus is limited because of the friction between the solid electrode and the metallic compound. Still in addition, since a certain mechanical strength should be maintained in the solid electrode, it is impossible to minimize it beyond a certain limit. For instance, according to the above conventional technique it is impossible to discolor selectively minute areas, for example, lines having a width of several microns.
I have conducted research with a view to developing a method according to which the foregoing defects of the conventional technique can be overcome and selected minute areas of a metal compound molded to have an optional form can be discolored at a high speed owing to the change in the composition of the metallic compound. As a result, it has been found that when a laser ray is radiated on the selected areas of the surface of a molded metallic compound, discoloration is caused at said selected areas at very high speed. It has been found that in order to accelerate the discoloration at irradiation of the laser ray and make it possible to use a laser device of a lower output, it is desirable to increase the sensitivity of the metal compound to laser rays and that in order to increase the sensitivity of the metallic compound to laser rays it is preferred that semiconductivity is given to the metallic compound to be treated.